Pliers are so universally used that almost all persons, from the housewife to the skilled artisan, use them from time-to-time. Because of the wide use of these tools, many modifications and variations have been made. They have varied from comparatively small tools with long slender jaws for delicate work to large locking and clamping tools for heavy duty machinery. Moreover, the jaws have taken many forms and shapes, including C-shaped jaw members, straight jaws, curved jaws, pinch-off jaws, elongated flat plate-like jaws for sheet metal work, welding clamp jaws, and movable jaw members coupled with a chain clamping means enabling a workpiece such as a pipe to be effectively gripped.
One of the purposes for which pliers are commonly used is for applying a torque to a workpiece such as a pipe or rod, either to cause the workpiece to rotate about its axis, or to resist rotation of the workpiece about its axis. To accomplish this, two kinds of force are required--a gripping force to grip the workpiece and a rotative force. Pliers used for this purpose preferably have teeth formed on a curved or arcuate jaw so that the opposing jaws provide curved portions for gripping a cylindrical workpiece, or for gripping corners of a polygonal workpiece. These curved portions generally have transverse gripping teeth to engage and bite into the workpiece when a gripping force is applied to the plier. Then when a rotative force is applied to the plier handle, the frictional resistance generated by the engagement of the teeth with the workpiece resists relative rotation so that a torsional force is applied to the workpiece. The greater the gripping force applied, the greater the frictional resistance, and the greater the torque which may be exerted to cause or to prevent rotation of the workpiece.
Pliers may be classified as either locking pliers or non-locking pliers. In a non-locking plier, the gripping force is applied by the hand of the user, and the amount that may be applied is directly related to the user's strength, whereas locking jaw pliers grip the workpiece with brute force generated and applied through the mechanical advantage of a locking toggle mechanism and frame. Once established, the grip is maintained as a result of the toggle mechanism locking past center, and the user of the plier can then apply a rotational force on the workpiece. The degree to which the tool grips, and is able to transfer torque to the workpiece, is highly dependent upon the design of the toggle mechanism of the tool. Locking pliers are shown for example in Petersen U.S. Pat. No. 2,563,267 and in Petersen U.S. Pat. No. 4,546,680. Such locking pliers are capable of applying a much higher gripping force than a user can apply to an ordinary plier, so a much higher torsional force can be applied to the workpiece before the plier slips.
When such prior art pliers are used to apply a torque to a cylindrical workpiece, it will be appreciated that the amount of torque generated is dependent in part on the amount of friction between the teeth of the plier and the workpiece. The application of force through the teeth causes them to bite into the workpiece, thereby causing small depressions in the workpiece to increase the amount of friction. The teeth in prior art pliers have generally been symmetrical, i.e. both faces of the teeth are at substantially the same angle with the surface of the work being gripped, with a total included angle of from about 80.degree. to about 100.degree.. Smaller angles are generally undesirable because the teeth are made weaker. Such jaw tooth designs grip and allow torsional load application equally in both directions due to the symmetrical tooth profile pattern. However, because of the large included angle which must be used to avoid weakened teeth, no more than about 70% of the force applied to the plier is applied radially to the work, with the remainder of the rotational force being applied tangentially, thereby tending to counteract the gripping force and induce slippage. Thus, with even moderate gripping pressures the teeth can cam or tear out of the surface of stubborn objects, causing damage to the workpiece or injury to the user.
To prevent such slippage the user of a locking plier adjusts the plier to grip tighter. This means he has to squeeze the plier handles together with more pressure to cause the handles to come together and the toggle mechanism to lock past center.
Asymmetrical teeth have previously been used on a straight jaw non-locking plier, as illustrated in the drawing at FIGS. 4 and 5, to reduce the tendency of the teeth to slip. However, at best a straight jaw plier can grip a cylindrical workpiece with only two teeth of each jaw, so a gripping force sufficient to prevent slippage, such as may be applied with locking pliers, may result in undesirable damage to the surface of the workpiece. In addition, the use of asymmetrical teeth on a straight jaw reduces the width of the base of the teeth, so that they are weaker than symmetrical teeth and are more likely to be damaged in use. This problem, also, is accentuated with the higher gripping force which is possible with locking pliers.